Ultrasound imaging systems have become an important diagnostic tool in many medical specialties. One important advantage of an ultrasound imaging system is real-time scanning. For example, an ultrasound imaging system can produce images so rapidly that a sonographer can scan internal organs or can discern motion within a body, such as blood flow, with real-time, interactive, visual feedback. This allows the sonographer to examine structures of interest and to modify the examination in real-time, thereby speeding and improving the diagnostic process and increasing patient throughput.
Along with the advantages of real-time, interactive, visual feedback, sonographers are still concerned with system resolution. In an ultrasound imaging system, system resolution depends on the system's ability to focus, which ability to focus depends, in turn, on the effective aperture of a transducer array in a probe associated with the ultrasound imaging system. Currently two types of arrangements of transducer array elements are used for real-time, ultrasound imaging systems. One type of arrangement of transducer array elements comprises a single transducer element or an annular array of transducer elements. Ultrasound imaging systems using this type of arrangement of transducer array elements rely on mechanical motion of the probe to sweep an acoustic beam over a region of interest. A second type of arrangement of transducer array elements comprises an array of transducer elements which is activated by electronic circuits which produce electronically induced time delays in the transducer element acoustic outputs, which phase delays cause the acoustic beam produced by the transducer array to be steered and/or focused.
Links between electronic circuits which generate transmit pulses for transducer array elements and the transducer array elements which receive the transmit pulses are referred to as beamformer channels. Electronic steering and/or focusing of an acoustic beam produced by the transducer array is achieved by electronically delaying transmit pulses, on a beamformer channel-by-beamformer channel basis, to create an effective lens having varying thickness. Due to limits on: (a) the size and complexity of a cable connecting the ultrasound probe with the processing system and (b) the number of beamformer channels available in a reasonably priced ultrasound system, electronic focusing has been limited to a lateral direction (a direction parallel to the imaging plane). Focusing in an elevation direction (a direction perpendicular to the imaging plane) has been accomplished by placing a mechanical lens, of fixed curvature, on the probe face. In the prior art, modifications in elevation focusing have been accomplished by changing the probe aperture and/or the mechanical lens. Although it is known that changing frequency can change focal depth (higher frequencies producing deeper focusing than lower frequencies), it is not considered advantageous to change frequency to change focal depth because higher frequencies are attenuated more rapidly in tissue than lower frequencies. Thus, it is known that in order to change elevation focusing of a transducer array, one ought to change the elevation aperture and/or to change the effective lens curvature of the transducer array. For example, in imaging a deep organ, the lens ought to have a large aperture and mild curvature and, in imaging a shallower object, the lens ought to have a smaller aperture and a tighter curvature.
As is known, transducer array elements in an ultrasound probe can be arranged in a 1-D array, a 1.5-D array, or a 2-D array (the size of a typical transducer array element is on the order of 0.5 wavelengths in the lateral direction and is on the order of 50 wavelengths in the elevation direction). In a 1-D array, all transducer elements are disposed in the lateral direction, with a single row of elements in the elevation direction. Conventional phase linear arrays and curved arrays are generally considered to be 1-D arrays. In a 2-D array, transducer elements are mounted in both the lateral and elevation directions, with electrical connections providing control and data to transducer elements in both directions. An acoustic beam produced by a 2-D array can be steered and focused in two dimensions. An example of a 2-D array ultrasound probe can be found in U.S. Pat. No. 5,186,175. In a 1.5-D array, transducer elements are also mounted in both the lateral and elevation directions, but control and data electrical connections are symmetrically connected about the elevation center so that an acoustic beam produced by a 1.5-D array can only be steered in the lateral direction.
The advantages of 2-D array imaging are well known. For example, such advantages include the ability to steer in two (2) dimensions, enhanced resolution due to improved elevation focusing, and improved phase aberration correction through refined comparison of propagation velocities. However, due to the: (a) lack of reliable interconnection technology; (b) limitations on cable size; and (c) limitations on the number of beamformer channels that are commercially viable, 2-D arrays are not considered to be practical yet.
FIG. 1 shows, in pictorial form, a conventional 1.5-D array. The notation N.times.M describes a transducer array having N lateral transducer elements per row and M transducer elements per column. Further, the notation n*m describes the electrical connections. For example, the 1.5-D array depicted in FIG. 1 is a 64.times.7 array having 64*4 channels. By grouping transducer elements in the elevation direction into a limited number of groups, and by activating specific ones of the groups when imaging at a specific depth, the 1.5-D array probe is capable of providing some depth dependent focusing in the elevation direction. However, such depth dependent focusing using a 1.5-D array probe suffers from a problem in that switching among the limited number of groups of transducer elements in the elevation direction to change elevation focus causes abrupt textural changes in a displayed image.
In light of the above, there is a need in the art for method and apparatus for controlling elevation focusing of a transducer array of an ultrasound probe by changing the elevation aperture of the transducer array and by smoothly varying elevation focus of transducer arrays having a limited number of transducer array elements in the elevation direction.